The problem of accurately determining the location of faults in underground cables (commonly termed "pin-pointing") has been studied for many years, resulting in a variety of proposed techniques. These methods generally rely on above-ground detection of one or more indicia, such as changes in voltage, current magnetic field, sound, or electromagnetic signals. Each of these methods has particular advantages and disadvantages, as noted below.
The most widespread methods for precisely locating faults in power cables are based on acoustic detection of an arc at the fault. Typically, a surge generator (sometimes termed a "thumper") is used to excite the cable with a series of high-energy pulses, which in turn prompt audible sparking and vibration at the fault. More particularly, a variable high-voltage D.C. source is connected to the cable through a high-voltage capacitor bank, with a timing circuit and solenoid- or thyristor-controlled switch to determine the voltage and rate of discharge.
When energy stored in the capacitor bank is discharged into the test cable, a high-voltage pulse travels down the cable towards the fault. This pulse typically has a steep leading edge and an exponentially decaying trailing edge. As the voltage at the fault rises, it eventually causes the fault to flash over, forming an ionized short between the two conductors. (The cable can be modelled as a distributed resistive/capacitive/inductive network, the time constants of which determine when the fault flashes over.)
The breakdown of the fault causes localized audio noise and/or vibration (sometimes termed a "thump"). However since the cable is buried, the sound is usually muffled so that a sensitive microphone and audio amplifier is often required for detection. In practice, the process of discharging and listening is typically repeated every few seconds while a technician slowly walks along the area under which the cable is buried.
Acoustic fault location suffers from a number of drawbacks. Primary among these is the simple difficulty of discerning the fault-induced "thumps" from other ambient noise, and accurately pin-pointing their origin along a buried cable. Echoes from rocks and other buried hard objects, for example, can readily produce misleading results. Media between the cable and the surface (e.g. plastic conduit, sandy soil, pavement) can effectively soundproof the cable. Yet another difficulty is the requirement of a highly skilled technician who is trained in the nuances of acoustic differentiation. (Electronic audio-based detectors are sometimes used in lieu of human locators, but are generally viewed as inferior to skilled human technicians.)
Another problem with acoustic detection techniques is that they rely on explosive arcs in order to produce sounds sufficiently loud to be detected above ground. To induce explosive arcing, the cable must be electrically abused with very large thumps, accelerating future failures of the cable.
While the discussion so far has focused on arcing faults, it should be noted that other types of faults, such as bolted faults and open faults, sometimes need to be located. Current practice with such faults is generally to abuse the cable sufficiently (e.g. by blasting or burning with repeated application of a large thumper voltage) that an arcing fault is finally produced, and then locate the arcing fault by traditional methods.
Another class of location techniques, albeit not suited for pinpointing, are those based on electromagnetic detection. In such systems, an operator moves a handheld electromagnetic detector along the buried cable, sensing the passage of large fault currents in the line below. Such techniques are generally used just to identify which of several lines has a fault.
Still another class of fault location methods is based on injection of an audio frequency signal into the cable, and its detection as it propagates along the cable.
More sophisticated than the foregoing techniques, and commensurately more complex, is arc reflection time-domain reflectometry (TDR). This technique induces an arc at the fault (i.e. by a surge generator connected at one end of the cable), and then introduces a series of brief, lower voltage pulses into the cable. These pulses encounter a transmission line discontinuity at the arc and reflect back to their origin where they are detected. By knowing the speed at which signals travel along the cable (the velocity factor), and by measuring the time it takes a pulse to make the round-trip, the distance to the fault can be determined.
The accuracy of arc reflection TDR is limited by the accuracy of the a priori information. For example, a small difference in the actual versus estimated velocity factor of the cable can result in a one to two percent difference between the best TDR estimate and the actual location of the fault. For a cable length of only 1000 feet this difference could be up to 20 feet. Further, there is often a difference between the distance measured above ground and the distance actually traversed by the cable, which leads to other inaccuracies--even ignoring uncertainty due to cable velocity factor. Therefore, the use of the arc reflection TDR method is limited to gross location of the faults (sometimes termed "pre-location"), after which a more accurate method must be used for pin-pointing the fault location.
Another difficulty with TDR systems arises when trouble-shooting a complex cable system containing many branches and other connections (as is often the case in practical application). In such cases the reflected waveforms can become very difficult--if not impossible--to analyze.
To help alleviate this latter problem, and to assist in fault location generally, power distribution systems typically include devices known as Fault Current Indicators (FCIs) at periodic spacings to indicate whether a fault current passed that point in the distribution system. By analyzing data from the FCIs in a given system, it is sometimes possible to identify a particular length of cable suspected to contain the fault. By isolating this cable from the rest of the network, TDR techniques can be used more successfully to pinpoint a fault. (Uncertainties due to velocity factor and above/below ground path differences remain.) FCI data, however, is generated by actual network power distribution (i.e. 60 Hz power), rather than thumper signal. To adequately localize a fault with FCIs so that TDR data can be effectively used often requires repeated full-power energization of the faulty network (i.e. reclosure of protective breakers or fuses), causing undesirable stress and wear on the system.
To more fully appreciate the background of the present invention, it is appropriate to turn from consideration of what artisans have done in this field to what they are doing.
For the past several years, and for the foreseeable future, the technologies that have captured the interest of artisans in this field are those based on digital signal processing- (DSP) and microprocessor-based analyses. With the advent of powerful, off-the-shelf DSP chips, engineers are able to apply tremendous computing resources to the task of fault location, readily applying numerical analysis techniques that were out of the question a few years earlier.
One such technique involves use of a thumper to excite a faulty cable. When the fault occurs, pulses propagating along the line reflect from the fault, causing the cable to ring with a plurality of superimposed waveforms. Previously, the resultant waveform was too complex to be used directly to show the fault location. Now, however, the pattern of ringing can be mathematically processed and analyzed to discern the approximate fault location that would produce that characteristic pattern.
While these mathematical model-based techniques hold great promise, they also have their drawbacks. In addition to the obvious concerns about cost and complexity, these systems rely on acquisition of data from the cable at a point remote from the fault. Thus, these systems must identify the location of a fault in terms of its spacing from the measurement point. However, as noted earlier, such calculations must be premised on knowledge of the cable's velocity factor, and if this number is in error by only a percent or two, the analysis will fail to provide the desired pinpointing information.
It is an object of the present invention to provide a fault pinpointing system that overcomes certain of these drawbacks of the prior art. In many embodiments of the invention, this is achieved by identifying a direction to the fault from a present location.
In a first exemplary embodiment, a surge generator is connected to one end of the cable and injects a high-energy pulse ("incident pulse") into the cable. After a brief interval (during which the pulse may propagate back and forth along the cable a few times), the pulse causes the fault to arc over, generating a pair of fault current surges that travel away from the fault in opposite directions.
A receiving device, including an electromagnetic detector (e.g. a loop antenna) and discrimination circuitry, detects the initial polarity of the resulting electromagnetic pulse and provides a corresponding indication to an operator. From this polarity indication, the operator knows the direction to the fault. By moving the receiving device along the path of the buried cable until the polarity indicator detects a change in polarity, the fault can be precisely located.
More particularly, the pair of initial steep, rising edges of the fault surges induce voltage pulses of opposite polarity in the loop antenna, because they are physically travelling in opposite directions. Ignoring the incident pulse and its reflections, the initial steep, rising edges of the pair of fault surges are the first electromagnetic events that occur after the fault breaks down. Thus, regardless of the complexity of the cable system and how many later reflections of the incident pulse and fault pulse surges occur, the polarity of the first fault surge detected by the receiver indicates the direction from the receiving device to the fault.
It should be recognized that devices according to the present invention can make their direction determination from a variety of different excitation signals--not just thumper signals. Other suitable signals include AC power, AC test, and DC test signals.
One advantageous application of applicant's invention is as a direction-indicating FCI. Such devices can be permanently installed at spaced locations in a power distribution system and--in the event of a fault--indicate to technicians the directions to the fault from each respective device. Instead of relying on thumpers, such devices work with the normal AC power signals.
In other embodiments, a fault-direction detection device according to the present invention can be combined with cable tracer for simultaneously tracing the path of the cable under test. In yet other embodiments, a fault-direction detection device can be combined with a time-domain reflectometer (pulse echo device) for prelocating the fault. In still other embodiments, a fault detection device can be combined with acoustic detectors or prior art electromagnetic sensors.
It will be recognized that prior art pin-pointing techniques have failed to appreciate that the polarity of an electromagnetic pulse generated by a fault current can be used to determine the direction to the fault. The present invention exploits this principle and provides a low cost technology for quickly and accurately pinpointing buried cable faults.
The foregoing and additional features and advantages of the present invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.